Seeds from space

Credit: NASA/JPL-Caltech/T. Pyle (SSC/Caltech)

Genesis: space-borne complex carbon molecules may have spawned life on Earth. 


Back in July 2010, the Spitzer Space Telescope made an astounding discovery. Scrutinising the warm cloud of gas and dust puffed out from a dying star, a planetary nebula called Tc-1, the infrared observatory spied the unmistakable signature of buckminsterfullerene molecules, or ‘Bucky Balls’. These are large compounds of carbon atoms arranged into three-dimensional structures: football-like spheres or more elongated like rugby balls. Just a few months later, in October 2010, Spitzer spotted their infrared signature drifting through nebulae in cold interstellar space.

Carbon chemistry forms the basis of all life as we know it, and these discoveries of remarkably complex organic molecules in outer space suggest a rather startling possibility. Could life on Earth have emerged from extraterrestrial molecules? A number of different organic molecules have already been discovered in the vast clouds of gas and dust in space, but Bucky Balls are by far the largest molecules now known to form in space. These are just the latest discoveries in an extraordinary detective story spanning the past few decades and part of an increasing realisation of just how important ‘astrochemistry’ is in producing organic molecules throughout the cosmos.

The Spitzer discovery of Bucky Balls is certainly a big leap forward in terms of the complexity of molecules that we now know can form in interstellar space. Bucky Balls have previously been discovered on Earth, formed by sooty, incomplete combustion, as well as within meteorites. While they’re not molecules directly used by life, the Bucky Ball structure of interlinked hexagons and pentagons of carbon atoms is very similar to key biological molecules like chlorophyll and haemoglobin. There is the possibility that these cosmic carbon footballs could become incorporated into the planet-forming dusty discs around newborn stars and delivered to virgin worlds within meteorites or comets, to play a key role in the origin of life.


What is an organic molecule

For starters, it’s got nothing to do with whether it was grown on a free-range farm without pesticides… to a chemist, ‘organic’ simply means a compound that contains carbon. Carbon is very good at forming bonds with lots of different atoms, and so the organic compounds are an incredibly diverse class of molecules.

From methane in the blue clouds of Neptune to the hydrocarbon smog of Titan, organic compounds are common in the Solar System. They are also the basis of all life as we know it: sugars, proteins and DNA are all organic molecules. Proteins are vital to life for everything from building the structure of cells to acting as catalysts (called enzymes) to drive biochemistry. Proteins are long, chain-like biological polymers, built up from small organic molecules called amino acids. DNA is another biological polymer, and it encodes the information of life in the sequence of nucleobase letters in its double-helix structure.

The versatility of organic chemistry and the realisation of how prevalent it is in the cosmos, give us confidence that any alien life is also likely to be carbon-based.


Cosmic compounds

The Spitzer discovery of Bucky Balls (see pic) was based on infrared spectroscopy, and similar searches using radio wavelengths have proved fabulously successful at spotting molecules in the circumstellar and interstellar medium. We now know of over 140 different molecules in space, many of them organic. The first organic molecule discovered in interstellar gas clouds was formaldehyde (H2CO; notable on Earth for its use as a preservative of biological specimens) in 1969. Since then we’ve detected ethanol (alcohol), acetic acid (vinegar) and glycolaldehyde (the simplest sugar molecule). The announcement in 2003 of the discovery of interstellar glycine, the simplest amino acid, was met with great excitement. Amino acids are, after all, the building blocks of proteins, but this claim remains controversial.

While the detection of biologically important molecules like amino acids remains fiendishly difficult in the diffuse clouds of interstellar space, the fact that they can form away from Earth is beyond doubt. In 1969 a lump of space rock fell to Earth near the town of Murchison, Australia, and was categorised as a rare carbon-rich meteorite called a carbonaceous chondrite. This Murchison meteorite has become one of the most intensely studied pieces of rock in history, and has been found to contain a staggering degree of carbon chemistry. It’s packed full of organics, including fullerenes (like the Spitzer discovery), alcohols, polar hydrocarbons (similar to the molecules that form our cell membranes) and over 70 kinds of amino acids, all of them extraterrestrial.

The combined results from interstellar spectroscopy and chemical analyses of carbonaceous meteorites like Murchison reveal the astounding complexity of astrochemistry in our Galaxy. Although all life on Earth is organic – based on the sturdy molecular frameworks of carbon atoms – other elements are also crucial, such as nitrogen and phosphorus. And perhaps key nitrogen-containing molecules were also delivered to Earth from space. This February, results were announced on experiments run on a meteorite found in Antarctica. After it had been treated with water at high temperature and pressure – conditions common on the primordial Earth – the meteorite released nitrogen-containing ammonia gas, which the researchers believe may have been important for the formation of the first biomolecules.

Perhaps the most exciting revelation about alien organics is that extraterrestrial amino acids show a bias towards the left-handed enantiomer (see ‘What is chirality?’ above). All life on Earth is exclusively built with left-handed amino acids, and the discovery of the same bias in extraterrestrial compounds is intriguing – it suggests that this quirk of life may have been inherited from outer space.


 
What is Chirality?

Many organic molecules possess a property known as chirality – they can exist in either of two mirror-image forms. Each mirror-image version is called an enantiomer, and one of the apparent quirks of life is that it only uses one particular enantiomer. With only very
few exceptions, all terrestrial life is based solely on left-handed amino acids and right-handed sugar molecules. So one of the best tests for whether organic compounds have been produced by life is to show that they are all of the same chirality.

But why is life so fussy? Well, the biological catalysts, called ‘enzymes’, that run all of the chemical reactions in our cells are unbelievably specific – they act upon only very few particular molecules. Their fussy nature is absolutely crucial for controlling and regulating the chemistry of life, and it means that enzymes can only work on one enantiomer. So once life developed a preference for, say, left-handed amino acids over the right-handed variety, it makes perfect sense in terms of efficiency to operate solely with that configuration.


An alternative theory: hypothermal vents

One theory recently gaining a lot of support is that life on Earth started in the dark depths of the seafloor, around a hydrothermal vent. The so-called ‘black-smokers’ are certainly impressive displays and have received a lot of attention, but the super-heated water they pump out is far too hot and acidic for life.

Far more promising are the much cooler alkaline vents, such as Lost City near the mid-Atlantic ridge. These vents aren’t found along the spreading centres of plate tectonics, but are driven by reactions of iron-rich rock in the Earth’s crust. The towering mineral chimneys formed by such vents last tens of thousands of years and are very porous, riddled with tiny cavities that can concentrate compounds and act as miniature reaction vessels. Most importantly, the difference between the alkaline vent fluids seeping out of the ocean floor and the slightly acidic seawater produces a gradient of pH, or protons. All organisms on Earth generate power using a similar proton gradient, and it seems as though the energy source of life could be inherited from our ancient crucible in alkaline hydrothermal vents.


The expert:

Dr Zita Martins of Imperial College, London discusses her work with the Murchison meteorite

What do you work on?

I analyse organic molecules present in carbonaceous meteorites, which are some of the most ancient objects in the Solar System and contain building blocks that may have contributed to the origin of life on Earth. I also develop methods of detecting signatures of past and present life. I work in a chemistry lab and so I use lots of different equipment and techniques, including mass spectroscopy and fluorescence detection, to figure out which molecules are present in samples.

What’s been your biggest discovery?

One of my biggest discoveries was to prove that nucleobases present in the Murchison meteorite were extraterrestrial. This shows that components of the genetic code were already present in our early Solar System.

How can you be sure these organic molecules were extraterrestrial?

We analyzed the carbon content of the meteoritic nucleobases. Carbon in molecules can be a lighter form (carbon 12) or a heavier form (carbon 13). Biological molecules formed on Earth consist of the lighter form. On the other hand, our analysis shows that the nucleobases in the Murchison meteorite were enriched in a heavy form of carbon that could only have been formed in space.

What does this mean for the possibility of life beyond Earth?

Between 4.6 and 3.8 billion years ago, a large number of meteorites bombarded the surface of planets like Earth and Mars. These contained the building blocks of cells (including nucleobases), and may have been important for the origin of life on Earth, and maybe in other parts of our Solar System.

What are you working on at the moment?

I have just analyzed some desert soils that host small amounts of living organisms and are Earth analogues for possible Martian life. They help us determine whether signatures of past and/or present life may still exist in the Martian soil. These studies also help when choosing target locations for future habitability and life-detection missions on Mars, such as the Mars Science Laboratory and ExoMars probes.


Left-hand life

What caused that selection of one enantiomer over the other in the first place? Over the years, a number of theories have been put forward as to what might naturally cause a bias towards left-handed amino acids in space. One long-running idea is that the ultraviolet light from hot young suns in a star-forming region becomes ‘circularly polarised’ and preferentially destroys right-handed amino acids.

Another theory, proposed in June 2010 by Richard Boyd, reckons that supernovae could be responsible. The essence of this new theory is that the surge of exotic radiation particles from an exploding star is more likely to interact with the nucleus of nitrogen atoms in right-handed amino acids, and so preferentially leave behind the left-handed variant. Incorporation of these organics into asteroids and comets, and subsequent delivery to the Earth may have driven developing life to adopt the same enantiomer preference. 

Whatever the cause of the curious enantiomer bias in extraterrestrial organics, the greater mystery is whether these molecules actually had any role to play in the emergence of life on Earth. The fact that organics can form in the cold of outer space, and are present in abundance in certain meteorites, is testament to the fact that carbon is just really good at chemistry. On a fresh, habitable world like the young Earth you’d also expect a great deal of organic chemistry going on in its own warm atmosphere and seas. The arrival of extra organics from space was probably a drop in the ocean – literally. Foremost among current theories as to where life got started is hydrothermal vents on the seafloor, which are thought to be able to synthesise a whole range of vital organic molecules and provide the essential energetics for life. If life did get started from chemistry driven in these deep-sea chimneys, it is hard to see how extraterrestrial organic molecules arriving on Earth, and being diluted in the entire ocean, could have been significant. It might be a simple cosmic coincidence that the chemistry of our cells shows the same enantiomer bias as organics in meteorites. 

It certainly is a very exciting time in astrochemistry and the analysis of organics from outer space. We’re only just now appreciating the full cornucopia of compounds that can form extraterrestrially, and may have played a role in the origins of life on Earth. The big question remains, though. How on Earth do you get from the basic building blocks of life such as amino acids and nucleobases to the incredible complexity of replicating cells?


This article first appeared in the May 2011 issue of Sky at Night magazine

Photo Credit: Thinkstock, Imperial College London 
 
 
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